At 146,000 miles, it should only need a top end rebuild, it depends on how it was driven over the years. Too many short trips around town and not having it fully warmed up may have worn the bores prematurely.
You can do a compression test with oil in the bores to determine where the compression issue is, but at 146K, if it's never had its valves done, it's time to do it.
Compression should be at 155/160.

That engine has quite a short stroke, it has no problem running at 70mph in over drive.
Mine happily revved to 5000 rpm, not that I would drive it like that all the time.
But in over drive at 70 mph, I think it was running at under 3000rpm _ certainly not being "taxed" that's for sure.
Getting off subject here, when you decide to go for it, start a new thread if you need help.